The present invention relates to the art of nanotechnology, and in particular, to carbon nanotube technology, its function and structure.
A carbon nanotube is a single graphene sheet in the form of a seamless cylinder. The ends of a nanotube typically have hemispherical caps. The typical diameter of a nanotube ranges from about 1 nm to 10 nm. The length of a nanotube potentially can be millions of times greater than its diameter.
Carbon nanotubes are comprised of shells of sp2-hybridized carbon atoms forming a hexagonal network that is itself arranged helically within the cylinder. Basically, helicity is the arrangement of the carbon hexagonal rings with respect to a defined axis of a tube. (M. S. Dresselhaus et al “Science of Fullerenes and Carbon Nanotubes” (Academic Press, New York, 1996)).
Carbon nanotubes are grown by combining a source of carbon with a catalytic nanostructured material such as iron or cobalt at elevated temperatures. At such temperatures, the catalyst has a high solubility for carbon. The carbon links up to form graphene and wraps around the catalyst to form a cylinder. Subsequent growth occurs from the further addition of carbon.
Since their discovery in the early 1990s, carbon nanotubes have been the focus of intense study due to their very desirable and unique combination of physical properties. They are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive. In fact, carbon nanotubes may potentially be stiffer and stronger than any other known material.
Carbon nanotubes are currently being proposed for numerous applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for the next generation of electronics devices.
There has been particularly intense study of the electrical properties of nanotubes, and their potential applications in electronics. Metallic carbon nanotubes have conductivities and current densities that meet or exceed the best metals; and semiconducting carbon nanotubes have mobilities and transconductance that meet or exceed the best semiconductors.
The physical properties of carbon nanotubes are structure-dependent. For example, depending on the diameter and helicity of a nanotube, the tube can be either metallic or semiconducting. Also, a single structural defect in a hexagonal ring can change a metallic nanotube to a semiconducting nanotube. Current methods for producing nanotubes results in a mixture of tubes with diverse diameters, helicities and structural defects. Thus, a mixture of metallic and semiconducting nanotubes are produced.
Separation of single-walled carbon nanotubes (SWNTs), according to their electronic properties, is essential to the development of molecular electronics, including field-effect transistors.
Techniques have recently been reported by which separation based on electronic properties, i.e., chiral separations, of SWNTs has been achieved. These techniques are associated with (i) alternating current dielectrophoresis and (ii) the current-induced oxidation of metallic nanotubes. (Collins et al., Science 2001, 292, 706; Krupke et al., Science 2003, 301, 344.) However, these techniques require individualization of nanotubes in order to evaluate their electronic properties, i.e., these methods cannot be applied to bulk samples. Instead, individual nanotubes have to be placed one by one between electrodes in a transistor geometry.
Noncovalent and covalent sidewall chemistry to probe differential reactivity in metallic and semiconducting nanotubes has been used to effect the bulk separation of nanotubes. (Chen et al., Nano Lett. 2003, 3, 1245; Chattopadhyay et al. J. Am. Chem. Soc. 2003, 125, 3370; Strano, M. S. J. Am. Chem. Soc. 2003, 125, 16148; Strano et al. Science 2003, 301, 1519.) However, such noncovalent methods are not efficient; and such covalent methods require nanotubes to be individualized.
Accordingly, there remains a need for a simple and efficient method of differentiating between semiconducting and metallic SWNTs which does not require individualization of SWNTs, i.e., which can be performed in bulk